then you have air pockets in the coolant circuit...especially possible as you have mentioned recent cooling system work. There has to be a bleed screw somewhere (I haven't had to do this on my 300TE yet so I don't know exactly), but I have done it on BMW, SAAB and Audi. Very much like like living in an older home with radiator heat and the pipes would clank and you have bleed the radiators in the house with this goofy little key and let the steam out.....close and it's all good....same thing sort of...